In diagnosis using a nuclear magnetic resonance, a required accuracy in a magnetic field intensity of the magnet system is such that displacement of one millionth in a magnetic field intensity is considered to be a problem because a magnetic field intensity corresponds to a diagnosis place. There are three types of magnetic fields in MRI apparatuses. They are a static magnetic field, a gradient magnetic field, and high frequency magnetic field. That is:
(1) A magnetic field (static magnetic field) that is constant in time base and uniform spatially, and has an intensity of generally more than 0.1 to several tesla and a displacement range of about several ppm within a space for imagining (a space of a sphere or an ellipsoid with a diameter of 30 to 40 cm);
(2) A magnetic field (gradient magnetic field) varying with a time constant of about one second or shorter and inclined spatially; and
(3) A magnetic field (high frequency magnetic field) caused by a high-frequency electromagnetic wave having a frequency (several MHz or higher) corresponding to the nuclear magnetic resonance.
The present invention is mainly related to the static magnetic field of (1), but in the magnetic resonance imaging apparatus, particularly in a region where tomographic imaging of a human body is performed, it is required that the distribution of the magnetic field intensity of the magnetic field is spatially constant in time base and uniformity with extremely high accuracy.
The high accuracy mentioned here is, for example, an imaging space Field of View (FOV) having a diameter of 40 cm, which requires an accuracy of a residual magnetic field of the order of one millionth such as ±1.5 ppm. In order to realize a magnetic field distribution requiring extremely high accuracy uniformity as described above, it is necessary to accurately adjust the magnetic field after production and excitation of a magnet.
Generally, an error magnetic field due to a production error is 1,000 times or greater a permissible error magnetic field required for a uniform magnetic field. Therefore, since the magnetic-field-adjustment (shimming) required at the time of installation after production is an operation to reduce the residual magnetic field (error magnetic field) from several 100 ppm to several ppm, a magnetic-field-adjustment apparatus and a method with extremely high accuracy are required.
As a related art, PTL 1 discloses a shimming method of performing arrangement calculation of a magnetic moment using singular value decomposition and implement based thereon. The method described herein is a method that uses censored singular value decomposition and a current potential, calculates the distribution of the magnetic moment or iron piece, and performs a shimming operation with a result thereof.
A mechanism (shimming mechanism) for implementing shimming by the method in the related art is shown in FIG. 2A. In the drawing, a magnetic field runs in a vertical direction on a paper surface, and an assumed magnetic resonance imaging apparatus is a vertical magnetic field type (open type) MRI apparatus.
Upper and lower circular faces are shim trays for arranging magnetic materials (shim pieces) such as iron pieces and magnet pieces, a shim piece to be arranged here flattens (shimming) the magnetic field distribution of a region of interest (VOI: Volume Of Interest, but here, a region to be subjected to magnetic field shimming including FOV) written with a spheroid at a central portion. That is, in PTL 1, a method of converting an obtained current distribution into a distribution of the magnetic moment and then converting into an amount of iron, a magnet or a small coil for shimming is described.
The outline of the calculation method in PTL 1 is as follows.
First, magnetism on the VOI is measured using a sensor (magnetic sensor) that measures magnetism, and a difference (error magnetic field) with a target error is obtained. Next, the distribution of the magnetic field that cancels the obtained error magnetic field is obtained from the shim piece on the shim tray by singular value decomposition on a response matrix to a large number (about several hundred points) of magnetic field evaluation points in the VOI region. Then, magnetic-field-adjustment (shimming) is performed using obtained eigen-modes (consisting of eigen-distribution functions of a shim piece distribution on the shim tray and the VOI magnetic field distribution, and singular values indicating the relationship therebetween). In the diagram presented in FIG. 2B, an error magnetic field component from a target magnetic field intensity of the VOI is decomposed into the intensity when represented by superposition of eigen-modes, and the singular values are sorted in order of magnitude and graphed. Among an eigen-mode group, it can be seen that an error magnetic field is generated in low-order (small singular value) eigen-modes. In PTL 1, large eigen-modes enclosed by a circle in FIG. 2B is selected and adjusted by magnetic-field-adjustment.
In addition, in NPL 1, in order to maintain the plasma within a predetermined space, a method of assuming a current plane consisting of a closed surface and determining a current distribution that reproduces the magnetic field on the plasma surface in a case where the plasma is confined in the inside surrounded by the closed surface is described. In NPL 1, a method of expressing a curved surface as a set of triangular elements and obtaining a current distribution that reproduces the magnetic field distribution given on the curved surface is described. The concept of grasping a curved surface as an aggregate of triangular elements is also common to PTL 1.